Brain aversion system

I 5-HT acts at different levels of the brain aversion system, inhibiting brainstem hardwired panic system but increasing anxiety in temporal lobe structures involved in condition/generalised anxiety. 

It has been known for many years that aversive stimuli increase serotonergic transmission (reviewed by Chaouloff 1993) and so it was inevitable that exaggerated serotonergic transmission in a hypothetical brain punishment system became linked with anxiety. Unfortunately, much of the evidence for this idea was gleaned from unreliable measures of changes in 5-HT concentration in rodent brain tissue postmortem after experience of moderately severe forms of stress in vivo. Nevertheless, this concept was encouraged by reports that a reduction in 5-HT transmission, following administration of either the 5-HT synthesis inhibitor, p-chloroamphetamine (pCPA),... 

Smokers who have been abstinent for several weeks, or those who smoke for the first time, often experience nausea even at low blood nicotine concentrations. This aversive effect is due to the action of the drug on the chemoreceptor trigger zone whereby it indirectly activates the release of dopamine. Apomorphine and related dopamine agonists also cause nausea by activating the dopaminergic system in this brain region. 

---